This invention relates generally to a method of manufacturing an electroluminescent compound semiconductor wafer, and, more particularly, to an improvement of a method of producing an epitaxial wafer having epitaxial layers each composed of a mixed crystal of two components including elements belonging to Groups III and V of the periodic table, respectively, from which and electroluminescent device may be produced.
Group III elements in the periodic table forming one of the components of the mixed crystal are, for instance, aluminum (Al), gallium (Ga), and indium (In). On the other hand, group V elements are, for instance, nitrogen (N), phosphorus (P), arsenic (As) and antimony (Sb).
The mixed crystal of the III-V semiconductor materials in this invention has a composition defined as a "mixed crystal" or "solid solution" in crystallography. One component of the mixed crystal includes one kind of group III element and at least two kinds of group V elements, such as gallium arsenide phosphide (GaAs.sub.(l-x) P.sub.x, 0&lt;x&lt;1), includes at least two kinds of group III elements and one kind of group V element such as gallium indium phosphide (Ga.sub.l-y In.sub.y P, 0&lt;y&lt;1), or includes at least two kinds of group III elements and at least two kinds of group V elements such as gallium indium arsenide phosphide (Ga.sub.(l-y) In.sub.y As(.sub.l-x) P.sub.x, 0&lt;x&lt;1, 0&lt;y&lt;1).
An epitaxially grown III-V semiconductor such as gallium arsenide phosphide (GaAs.sub.l-x P.sub.x, 0&lt;x&lt;1) or gallium indium phosphide (Ga.sub.y In.sub.l-y P, 0&lt;y&lt;1) can be relatively freely changed in the peak light emission wavelength in the infrared and visible ranges by setting the mixed crystal ratio (x or y) to a suitable value. The epitaxially grown III-V semiconductors are extensively employed to manufacture light emitting diodes.
That is, the semiconductor wafer for diodes for red emission is obtained by epitaxially growing a GaAs.sub.l-x P.sub.x layer on a GaAs monocrystal substrate with x=0.40, the semiconductor wafer for diodes for orange and yellow emissions are obtained by epitaxially growing a GaAs.sub.l-x P.sub.x layer on GaP monocrystal substrates with x=0.65 and X=0.85, respectively, and the semiconductor wafer for diodes for green emission is obtained by epitaxially growing a GaP layer on a GaP monocrystal substrates. Alternatively it may be obtained by epitaxially growing a III-V mixed crystal on a monocrystalline substrate of Ge etc. which is a Group IV semiconductor.
As conducive to a full understanding of the above-described conventional epitaxial wafers, a description will be made in detail with reference to the accompanying drawing.
FIGS. 1 and 2 are a sectional view of an epitaxial wafer for red emission diodes, and an epitaxial wafer for yellow emission diodes, which are manufactured according to the conventional method. Referring to FIG. 1, reference character 10 designates a monocrystal substrate which may be a monocrystalline substrate of a Group III-V semiconductor such as gallium arsenide, gallium phosphite etc. or a monocrystalline substrate of IV semiconductor such as Si, Ge etc., the thickness of the substrate being in a range from 100 .mu.m to 1000 .mu.m and, preferably, in a range from 300 .mu.m to 450 .mu.m. In this example, the thickness is 300 .mu.m.
An epitaxial GaAs.sub.l-x P.sub.x region is made up of two layers 12 and 14. More specifically, the layer 12 is an epitaxial GaAs.sub.l-x P.sub.x layer referred to herein as a transition layer whose mixed crystal ratio x is gradually increased from 0 to about 0.4, while the layer 14 is an epitaxial GaAs.sub.l-x P.sub.x layer whose mixed crystal ratio x is constant, or about 0.40. On the other hand, referring to FIG. 2, reference character 20 designates a GaP monocrystal substrate, and an epitaxial region is made up of four layers 22, 24, 26 and 28. More specifically, the layer 22 is a GaP homoepitaxial layer, the layer 24 is a GaAs.sub.l-x P.sub.x transition layer whose mixed crystal ratio x is gradually decreased from 1.0 to about 0.85, the layer 26 is a first GaAs.sub.l-x P.sub.x constant layer whose mixed crystal ratio is constant, or about 0.85, and the layer 28 is a second GaAs.sub.l-x P.sub.x constant layer whose mixed crystal ratio x is constant, or about 0.85. The layer 28 is similar to the layer 26, but has added with nitrogen (N) as an iso-electronic trap in order to improve the light emission efficiency.
With the epitaxial wafers for light emitting diodes manufactured according to the conventional epitaxial methods, the brightness values of the light emitting diodes manufactured from these epitaxial wafers are low in comparison with the theoritical value, and are therefor not always satisfactory as opto-electronic devices. Recent trends in the design of integrated circuits (IC or LSI) have been directed to the minimization of power consumption. This may be achieved by improving the light emission efficiency of epitaxially prepared diodes.